1. Field of the Invention
This invention relates to a solder composition, and more particularly to a solder composition which is adapted to be conveniently used for mounting of electronic circuit elements on a printed circuit board.
2. Description of the Prior Art
In the art, there has been conventionally known a solder material which is capable of being melted at a relatively low temperature of about 130.degree. C. or below. However, a remelting point of the conventional solder is as low as about 130.degree. C. or below, so that it is substantially impossible to use the solder material for mounting of electronic circuit elements on a printed circuit board, because the circuit elements generally generate heat sufficient to cause the solder to be readily remelted during operation.
In view of the above, in practice, a solder material of a melting point between about 250.degree. C. to 270.degree. C. has been used for mounting of electronic circuit elements on a printed circuit board. More particularly, the mounting has been typically carried out in a manner to immerse a printed circuit board having electronic circuit elements temporarily placed thereon in a tank in which a melt of a solder material having a melting point between about 250.degree. C. to 270.degree. C. is spouted to flow, to thereby substantially mount the electronic circuit elements on the printed circuit board through the solder material. Also, a printed circuit board which has a cream-like solder previously applied thereto and has electronic circuit elements temporarily placed on the cream-like solder layer is prepared and then passed through an infrared irradiation oven at about 210.degree. C. to 240.degree. C. or a vapor atmosphere of an inert solvent of a high boiling temperature, thereby substantially mounting the electronic circuit elements on the printed circuit board through the solder layer.
However, the soldering methods as described above have no application to the mounting of electronic circuit elements inferior in thermal resistance on the printed circuit board. Accordingly, in the case where electronic circuit elements inferior in thermal resistance are to be mounted on a printed circuit board, it has been carried out to manually mount the electronic circuit elements on the printed circuit board after application of a solder material having a melting point of a relatively low temperature to the printed circuit board. Also, in the case where it is necessary to firstly mount electronic circuit elements on one surface of a printed circuit board via a solder material and secondly mount electronic circuit elements on the other surface of the printed circuit board via a solder material, in the first and second steps it is required to use a solder material different in a remelting point, namely, it is necessary that the solder material which is applied in the first step is higher in a remelting point than that which is applied in the second step. Otherwise, when the second step is carried out, the solder material having soldered the electronic circuit elements onto the one surface of the printed circuit board is remelted. The varying of a solder material, every time the second step is carried out, causes the mounting operation to be highly complicated.